1. Field of the Invention
The present invention relates generally to optical pickup devices and particularly to optical pickup devices optically recording information on and reproducing information from a plurality of different types of recording media having guide grooves, respectively, different in pitch.
2. Description of the Background Art
In recent years, optical disks, optical cards and similar recording media can record large amounts of information signals densely and are accordingly utilized in audio equipment, video equipment, computers and other equipment.
Recently, in particular, for motion video information and the like used for example in computers, the data that are handled are drastically increasing in amount. Accordingly, a recording pit reduced in size, a guide groove reduced in pitch and the like are provided to provide optical disks with large capacities.
The above described optical disks and similar recording media have an information signal recorded therein in microns. Accordingly, reproducing such signal therefrom entails causing a beam of light to track a recording guide groove precisely. A variety of methods of detecting a tracking error signal is known.
For example, Japanese Patent Laying-Open No. 61-094246 describes a differential push-pull (DPP) method employing three beams including a main beam and two sub beams. The DPP method is generally employed for a variety of recording optical disks including compact disk-recordable/rewritable (CD-R/RW).
The DPP method, however, requires that relative to a guide groove at which the main beam is positioned, the sub beams be positioned offset in the radial direction of an optical disk by ½ of the pitch of the guide groove. This provides a tracking error signal degraded for a different type of optical disk having a guide groove different in pitch.
As a means for resolving such problem, Japanese Patent Laying-Open No. 09-081942 proposes a method, as will be described hereinafter with reference to FIGS. 11-14.
FIG. 11 schematically shows a structure of an optical system of a conventional optical pickup device 100.
With reference to the figure, optical pickup device 100 includes a semiconductor laser 1, a collimator lens 2, a diffraction grating 300A, a beam splitter 4, an objective lens 5, a condenser lens 7, and a photoreceptive unit 8 including photoreceptive elements 8A-8C each divided into two regions. Optical pickup device 100 records information on and reproduces information from an optical disk 6 having a guide groove 61.
Semiconductor laser 1 emits a beam of light 30P which is received by collimator lens 2 and collimated thereby. The collimated light is divided by diffraction grating 300A serving as an optical branching element into a main beam 30 for recording and reproduction and detecting a servo signal and two sub beams 31 and 32 for tracking. The three beams 30-32 are transmitted through beam splitter 4 and condensed by objective lens 5 on a recording medium or optical disk 6 at guide groove 61.
The three beams of light 30-32 are reflected and again transmitted through objective lens 5, reflected by beam splitter 4 and incident through condenser lens 7 on the three photoreceptive elements 8A-8C, respectively, which detect a push pull signal MPP of main beam 30 and push pull signals SPP1 and SPP2 of sub beams 31 and 32, respectively. Optical pickup device 100, as well as the conventional DPP method, obtains a tracking error signal TR by the following operation:TR=MPP−k·(SPP1+SPP2),wherein k represents a coefficient correcting a difference in quantity of light between main beam 30 and sub beams 31 and 32. Furthermore, k·(SPP1+SPP2) will also be indicated as a composite sub beam push pull signal SPP.
The conventional optical pickup device 100 shown in FIG. 11 is characterized by a periodical structure that diffraction grating 300A has. This feature will now be described with reference to FIGS. 12-14.
FIG. 12 is a perspective view of the structure of diffraction grating 300A in optical pickup device 100 shown in FIG. 11.
As shown in FIG. 12, diffraction grating 300A is divided into a first region 300a and a second region 300b by a line extending in a direction Y corresponding to that of guide groove 61 of optical disk 6. These regions each have a structure having a protrusion and a depression extending in a direction X perpendicular to that of guide groove 61 and periodically repeated such that the periodical structures have phases, respectively, different from each other by 180°.
FIG. 13 shows a position of main beam 30 and sub beams 31 and 32 condensed on optical disk 6 at guide groove 61 for optical pickup device 100 shown in FIG. 11.
As shown in FIG. 13, sub beams 31 and 32 diffracted by the periodical structure of diffraction grating 300A each have on optical disk 6 at guide groove 61 an optical phase difference of 180° in a half thereof. Consequently, sub beam 31 is divided into spots of condensed light 31m and 31n and sub beam 32 is divided into spots of condensed light 32m and 32n. Sub beams 31 and 32 each thus form spots of condensed light having two peaks, respectively, in intensity.
FIG. 14 represents in waveform push pull signals MPP, SPP, SPP1, SPP2 corresponding to a structure of optical disk 6 for optical pickup device 100 shown in FIG. 11.
As shown in the figure, sub beams 31 and 32 provide push pull signals SPP1 and SPP2 18020 out of phase with push pull signal MPP of main beam 30. Similarly, composite sub beam push pull signal SPP has a waveform 180° out of phase, in opposite phase, with push pull signal MPP of main beam 30.
As such if sub beams 31 and 32 are positioned on the same guide groove 61 as main beam 30, as shown in FIG. 13, tracking error signal TR as intended is obtained. Optical pickup device 100 shown in FIG. 11 can thus accommodate a different type of optical disk having a guide groove different in pitch from optical disk 6.
The above described DPP system, however, has a significant disadvantage in practical use. Currently increasingly used digital versatile disks (DVDs) include DVD-R/RW (having a storage capacity of 4.7 GB and a guide groove with a pitch of 0.74 μm), DVD-Random Access Memory (DVD-RAM) 1 (having a storage capacity of 2.6 GB and a guide groove with a pitch of 1.48 μm), DVD-RAM2 (having a storage capacity of 4.7 GB and a guide groove with a pitch of 1.23 μm), and the like.
If such a variety of DVDs is subjected to the system disclosed in Japanese Patent Laying-Open No. 09-081942, and objective lens 5 shifts in a direction (X) orthogonal to guide groove 61 of optical disk 6, then, tracking error signal TR would significantly be reduced in amplitude as compared with that provided in the conventional DPP method. Such reduction in amplitude is significant particularly for DVD-RAM1, DVD-RAM2, and other similar disks having guide grooves large in pitch.
An arithmetic circuit, a control circuit and the like for tracking error signal TR receives a signal, which in general is limited in how it varies in amplitude, and if tracking error signal TR significantly varies in amplitude as the objective lens shifts, optical pickup device 100 is significantly reduced in range of practical use.
The above disadvantage is resolved by a method, as disclosed in Japanese Patent Laying-Open No. 2004-145915. The publication describes an optical pickup device basically identical to the optical system of optical pickup device 100 shown in FIG. 11 except that diffraction grating 300A serving as an optical branching element is replaced with a diffraction grating 300B having a feature as will be described hereinafter with reference to FIG. 15.
FIG. 15 is a perspective view of a structure of diffraction grating 300B serving as another example of diffraction grating 300A in optical pickup device 100 of FIG. 11.
As shown in the figure, diffraction grating 300B has constantly periodically repeated grating grooves formed therein and is divided into at least three regions, i.e., first, second and third regions 300a, 300b and 300c, by a line extending in a direction (Y) orthogonal to that of the grating grooves.
While the second region 300b has a periodical structure with a phase similarly as described for diffraction grating 300A of Japanese Patent Laying-Open No. 09-081942, i.e., 180° out of phase with the first region 100a, the third region 300c intermediate between the first and second regions 300a and 300b is structured to be 90° out of phase with the first region 300a. 
Diffraction grating 300B thus structured can also prevent tracking error signal TR significantly reduced in amplitude as the objective lens shifts for example for DVD-RAM1 and DVD-RAM2 having guide grooves with large pitches. Optical pickup device 100 can thus be increased in range of practical use.
Japanese Patent Laying-Open No. 2004-145915 describes that diffraction grating 300B divided in three has each region having a periodical structure with a phase offset by an amount limited to 90° and 180°.
If a diffraction grating has a periodical structure positionally offset between adjacent regions to add a phase difference to diffracted light (or a sub beam), the diffraction grating, having a limited number of grating lines providing protrusions and depressions, cannot provide accurate phase variation at a boundary region. In that case, there is also caused unwanted high-order diffracted light, and hence reduced efficiency in utilization of light and unwanted light acting as noise. Accordingly, achieving an object with a minimized phase difference is desired.
Furthermore, diffraction grating 300B that has regions having the periodical structure with their respective phases offset by amounts limited to 180° and 90° has the intermediate or third regions 300c limited in optimum width and hence a reduced degree of freedom in design. Japanese Patent Laying-Open No. 2004-145915 is silent on a combination of phase offsets other than other than 180° and 90°, and any relationship between the direction of the grating grooves of the diffraction grating and a phase difference given.